JP2018531427A6 - Optical system for thermal imager - Google Patents

Abstract

The present invention includes two symmetric concave mirrors (20a, 20b) arranged in the same plane and having parallel optical axes (Oa, Ob), and arranged in front of the mirrors on the optical axes of the two mirrors, respectively. An optical system having a mirror for an image sensor comprising an image sensor array (24) having two adjacent edges that are substantially adjacent. An image sensor can be attached to the opacity mask (28), which is included in the periphery of the image sensor, in the surface of the mirror that extends beyond the image sensor, in front of each mirror (20). It has an entrance pupil (26).
[Selection] Figure 2

Description

  The present invention relates to thermal imagers, and in particular to optical systems adapted to such imagers.

  A thermal imager may include an image sensor array that senses wavelengths in excess of 2 μm provided with an optical system for focusing the image on the sensor. The optical system may have a configuration similar to a lens for visible radiation, except that the lens uses a material that transmits thermal radiation. Such materials are expensive and generally have low transmittance.

  FIG. 1 is a schematic cross-sectional view of an exemplary low cost optical system adapted for thermal radiation as described in patent application WO2002-063872. The optical system includes a mirror arranged in a Gregory telescope configuration. Light rays from the observed scene reach the concave primary mirror 10 (usually a paraboloid) and are reflected to the secondary mirror 12 (generally an elliptical concave surface). The mirror 12 reflects the light beam to the image sensor 14 located behind the central opening of the main mirror 10.

  The secondary mirror 12 is located between the scene and the primary mirror 10. This mirror is attached to a support 16 that filters incident radiation. The support 16 is highly permeable to heat rays so as not to impair the sensitivity of the imager.

  Since this optical system has a telescope configuration, it has a narrow field of view and is not suitable for indoor scenes.

  An image having two symmetric concave mirrors that lie in the same plane and have parallel optical axes, and two opposite edges that are located in front of the mirrors and are substantially adjacent to the optical axes of the two mirrors, respectively. An optical system for a thermal imager comprising a sensor array is generally provided.

  The image sensor may be attached to an opacity mask, the opacity mask comprising an entrance pupil in front of each mirror that is contained in the periphery of the image sensor and within a mirror surface that extends beyond the image sensor.

  Each pupil and the corresponding mirror reflects light rays parallel to the optical axis that pass through the pupil to the mirror and are reflected toward the nearest edge of the image sensor and through the pupil to the mirror edge under the image sensor. The light beam at a critical angle may be configured to be reflected toward the symmetry axis of the image sensor.

  The pupils may each be adjacent to the optical axis.

  The mirror may have substantially the same form factor as the optical sensor and may have an elliptical surface.

  The optical system includes four concave mirrors with parallel optical axes configured in four adjacent quadrants, wherein the four corners of the image sensor are substantially adjacent to the four optical axes, respectively. And four entrance pupils respectively located at four corners of the image sensor.

  Other advantages and features will become more apparent from the following description of specific embodiments of the invention, which are provided for purposes of illustration only and are represented in the accompanying drawings.

1 is a schematic cross-sectional view of a conventional optical system having a mirror for a thermal imager as described above. FIG. 1 is a schematic cross-sectional view of one embodiment of a wide-field optical system using a mirror. 1 is a schematic front view of one embodiment of a wide field optical system using a mirror. FIG. FIG. 4 is a perspective view of the optical system of FIG. 3. It is a figure which shows the example of the conversion of the image projected on the image sensor array by the optical system of FIG. 3, and the image from the viewpoint of the process. It is a figure which shows the example of the conversion of the image projected on the image sensor array by the optical system of FIG. 3, and the image from the viewpoint of the process.

  In FIG. 2, a symmetrical assembly of two optical subsystems using mirrors forms one embodiment of a wide field optical system. The two optical subsystem mirrors 20a and 20b are concave and have parallel optical axes Oa and Ob that are oriented towards the scene being viewed. The two mirrors may be adjacent along a common ridge 22 that is in the same plane and is located in the plane of symmetry of the optical system.

  The image sensor array 24 is located in a plane parallel to the mirror plane between the mirror and the scene and is offset with respect to the optical axis. The sensor 24 overlies the ridge 22 and preferably reaches the two optical axes as shown. The position of the sensor plane with respect to the focal plane of the mirror determines the focal length. The focal plane passes through the optical focus Fa and Fb of the mirror. For distant subjects, the focal plane and the sensor plane will be assimilated. To obtain a substantially clear image with a fixed focus optical system for a subject located several meters away, such as in a monitored room, the plane of the sensor is offset towards the scene with respect to the focal plane. Good.

  With this configuration, as illustrated with respect to the mirror 20a, incident light directed along the optical axis Oa passes through the nearest edge of the image sensor 24 to reach the center of the mirror and is aligned with the optical axis. Reflected to the edge of the sensor. An incident ray r1 parallel to the optical axis Oa and offset from the edge of the sensor 24 passes through the focal point Fa and hits the vicinity of the sensor edge.

  For clarity of disclosure, the edge of the physical image sensor is considered to coincide with the edge of the sensing area of the sensor. In practice, the sensing area may recede from the sensor edge. The principle described here actually applies to the sensing area of the sensor.

  The oblique ray r2 hitting the common ridge 22 is reflected at an angle depending on the incident angle of the ray on the mirror 20a. The illustrated ray r2 defines the optical axis Oa and the field of view of the optical subsystem, i.e. the ray r2 has the largest angle among the rays reflected toward the sensor by the mirror 20a.

  In this configuration, as shown, the ray r2 is preferably reflected toward the axis of symmetry of the sensor 24. Next, any ray that strikes the ridge 22 at an angle smaller than the angle of ray r2, such as ray r3, is reflected towards the same upper half of sensor 24. This constraint can be satisfied, for example, by an elliptical mirror adapted to the dimensions of the optical system.

  Light that reaches the ridge 22 at an angle that is greater than the angle of the critical ray r2 will be reflected toward the second, lower half of the sensor 24. This is not desirable because the second half of the sensor is used symmetrically by the second optical subsystem associated with mirror 20b. To block such rays, an off-axis entrance pupil 26a in the form of a suitably sized orifice formed in a mask 28 that is opaque to the radiation used may be provided. In this case, a symmetrical pupil 26b is provided for the second optical subsystem.

  The mask 28 may be arranged in a large latitude along the optical axis, and the size and position of the pupil 26a is determined by the bus formed by the optical axis Oa and the critical ray r2. Preferably, as shown, the mask 28 is positioned in the plane of the image sensor 24 so that it can serve directly as a support for mounting the sensor.

  The pupil 26a does not block oblique rays that cross the ridge 22 and reach the second mirror 20b. Such light rays are reflected to the outside of the sensor 24 by the mirror 20b, and thus do not affect the imager.

  By associating two off-axis symmetric optical subsystems in this way, the imager field of view can be doubled in the plane of the optical axis. To double the field of view in all directions, four off-axis optical subsystems may be assembled together as described below.

  FIG. 3 is a schematic front view of one embodiment of an optical system having a wide field of view in all directions. Four concave mirrors 20a to 20d having parallel optical axes are configured in four adjacent quadrants Q1 to Q4. Image sensor 24 may be centered over four quadrants, the four corners of which are preferably adjacent to the four optical axes of the mirror, respectively. The mirrors may be adjacent along a ridge that has the same form factor as the sensor and is contained within two orthogonal planes of symmetry of the optical system. The mirror and sensor are square here, but can also be rectangular.

  Four entrance pupils 26a-26d are associated with four mirrors 20a-20d, respectively. The pupil may be adjacent to each of the four optical axes, which in this embodiment are themselves adjacent to the four corners of the sensor 24. The pupil 26 is further on the diagonal of the image sensor, so FIG. 2 can be considered as a cross-sectional view along the diagonal of the system of FIG.

  The pupil 26 is illustrated in a circular form. They can also be rectangular with the same form factor as the image sensor. However, the circular pupil acts as a diaphragm, and the pupil diameter, which depends on the position of the pupil along the optical axis, affects the depth of field of the optical system and the amount of radiation transmitted to the sensor. Preferably, as shown, each pupil is contained within a specular region extending beyond the image sensor. With this arrangement, all rays passing through the pupil, parallel to the optical axis, reach the mirror.

  The dotted line area corresponds to the image projected onto the plane of the image sensor 24 by the pupils 26a and 26d. These images are substantially circles cut off at the symmetry axis that delimits the quadrant of the image sensor. The diameter of the cut circle is in principle equal to half the diagonal of the sensor, so the diagonal limit ray (r2 in FIG. 2) reaching the common point between the four mirrors is reflected towards the center of the image sensor. Is done.

  The quality of the mirror surface at the adjacent ridge determines the quality of the cut edge of the circle image. In practice, it is difficult to produce a ridge of constant quality. Thus, an image formed in the four quadrants may have a blurred edge along the axis of symmetry of the sensor. This is not a problem as will be disclosed later.

  FIG. 4 is a perspective view of the four-quadrant optical system of FIG. This figure shows the mask 28 not illustrated in the diagram of FIG. 3 in the foreground. Several elements of the sensor are shown through the mask 28. Since the mask 28 may have a certain thickness that helps to ensure stable support of the image sensor 24, the entrance pupil 26 is due to the bus formed by the critical ray r2 (FIG. 2) corresponding to the optical axis. According to a defined cone, preferably frustoconical. If they are not exactly frustoconical, the pupil may be formed by several cylindrical parts with different radii close to the frustoconical shape.

  5A and 5B show an example of an image projected by the optical system of FIG. 3 or 4 on the image sensor 24 and an image conversion in terms of its processing. The subject to be seen is a circle placed in the center of the imager's field of view.

  As illustrated in FIG. 2 for the two-mirror optical system, rays originating from the center of the scene seen and parallel to the optical axis are reflected towards the edge of the sensor, while rays coming from the edge of the scene are Recall that it is reflected towards the center of the sensor. Thus, the center of the scene is reflected towards the edge of the sensor and the edge of the scene is reflected towards the center of the sensor. Thus, by exchanging each half image generated by the two halves of the image sensor, a useful final image is obtained.

  In FIG. 5A, in a four mirror optical system of the type of FIGS. 3 and 4, the center of the scene is reflected towards the corner of the sensor and the corner of the scene is reflected towards the center of the sensor. Thus, the circle at the center of the field of view is sensed by the image sensor as quadrants at each of the four corners of the sensor as shown.

  In FIG. 5B, the four quadrants of the image provided by the sensor are swapped diagonally as indicated by the arrows in FIG. 5A to reconstruct the usable image of the circle. Therefore, quadrant Q1 is exchanged with quadrant Q3, and quadrant Q2 is exchanged with quadrant Q4.

  Thus, the final image edge receives the portion originally located on the axis of symmetry of the sensor, i.e. the portion formed by the light reflected by the ridge between adjacent mirrors, which is the surface quality of the ridge. May deteriorate. Therefore, imperfections due to ridges are seen at the edges of the final image, and in fact the edges do not convey much useful information.

  The center of the final image has a blind zone corresponding to the part hidden by the sensor. However, this blind zone is defined between rays that run parallel to the optical axis, so that the blind zone corresponds to a projected zone of the size of the sensor on the subject in the center of the field of view. If the object is far enough, the projected zone is much smaller than the sensor pixels and can therefore be completely undetectable.

  By way of example only, the imager was realized with an elliptical mirror having a conic constant of 0.199 and a radius of curvature of 12.067 mm with a field of view of about 80 °. The mirror and image sensor array had the same diagonal of about 13.6 mm. The image sensor was placed in the optical focal plane of the mirror, approximately 5.7 mm from the elliptical depression. The pupil had a diameter of 3.8 mm. With these dimensions, satisfactory clear images from 0.2 to 20 meters could be obtained.

  Many variations and modifications of the embodiments described herein will be apparent to those skilled in the art. For example, the mirrors need not be in contact with each other. There may be a gap between the edges of two adjacent mirrors, which results in a central zone without image sensor information. This band corresponding to the edge of the image generally does not convey useful information.

  Instead of providing a single image sensor covering all four quadrants, an independent image sensor may be provided for each quadrant, and this strategy would be more expensive than providing a single sensor.

  Preferably, the edge of the sensor, or more precisely the edge of the sensing area of the sensor, is adjacent to the optical axis. Of course, this configuration can comply with tolerance limits. If the edge is set back from the optical axis, information may be lost in the central band of the field of view. If the edge protrudes beyond the optical axis, the protruding portion of the sensor is not illuminated, resulting in a black band in the center of the reconstructed image. This latter case is preferred over the former case because the black band can be removed by post-processing the image and there is no loss of information.

Claims (5)

An optical system for a thermal imager,
Two symmetrical concave mirrors (20a, 20b) located in the same plane and having parallel optical axes (Oa, Ob);
An image sensor array (24) located in front of the mirror and having two opposite edges each substantially adjacent to the optical axis of the two mirrors;
An opaque mask (28) to which the image sensor (24) is mounted, wherein the mask is included within the surface of the mirror extending around the image sensor and beyond the image sensor; An opaque mask (28) with an entrance pupil (26) in front of (20);
An optical system comprising: Each pupil (26) and corresponding mirror (20)
A ray (r1) parallel to the optical axis reaching the mirror through the pupil (26) is reflected towards the nearest edge of the image sensor;
A ray (r2) at a critical angle passing through the pupil and reaching the edge (22) of the mirror under the image sensor is reflected towards the axis of symmetry of the image sensor;
The optical system of claim 1, configured as follows.   The optical system according to claim 2, wherein the pupils (26) are respectively adjacent to the optical axis.   The optical system of claim 2, wherein the mirror (20) has substantially the same form factor as the optical sensor and has an elliptical surface. Four concave mirrors (20a-20d) with parallel optical axes configured in four adjacent quadrants, wherein the four corners of the image sensor (24) are substantially in the four optical axes, respectively. A concave mirror (20a-20d) adjacent to
Four entrance pupils (26a-26d) respectively located at four corners of the image sensor;
The optical system according to claim 1, comprising:

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